1. Field of the Invention
The present invention relates to a method for producing a photocoupler used for introducing light to a waveguide device and a photocoupler produced by the same.
2. Description of the Related Art
Light input to a prism coupler is performed utilizing the phenomenon that, in the case of a prism coupler including a prism bonded to a waveguide, for example, when light is incident on the boundary between an area where a prism is existent and an area where no prism is existent (such a border will be referred to as the "edge surface"), the light incident in a tunneling effect manner on the area where the prism is existent is propagated through the area where no prism is existent without outgoing from the waveguide.
By such a method, however, the prism coupler cannot be integrally produced with the waveguide device. In order to realize integral production of a prism coupler with a waveguide device, a method of bonding a prism on the waveguide has been proposed (see, for example, Japanese Laid-Open Publication No. 4-159503). FIG. 11A is a cross-sectional view of such a conventional photocoupler, and FIG. 11B is an enlarged view of an edge surface e shown in FIG. 11A. As shown in FIG. 11A, the conventional photocoupler includes a substrate 111, a waveguide layer 112 provided on the substrate 111, and an equivalent gap layer 113 provided on the waveguide layer 112. The equivalent gap layer 113 has a refractive index lower than that of the waveguide layer 112. A prism 114 formed of a dielectric material is bonded to the equivalent gap layer 113 by a photocurable adhesive 115. The prism 114 and the photocurable adhesive 115 each have a refractive index higher than that of the waveguide layer 112. The prism 114 is secured to the equivalent gap layer 113 by, for example, applying the photocurable adhesive 115 to a bonding face of the prism 114, next pressing the bonding face to the equivalent gap layer 113 and then radiating light through a slanting face of the prism 114 to cure the photocurable adhesive 115.
In such a structure, light incident on the prism 114 is transmitted through the bonding face of the prism 114 to the equivalent gap layer 113 in a tunneling effect manner and propagated through the waveguide layer 112. As shown in FIG. 11B, the edge surface e of the photocurable adhesive 115 does not cross the top surface of the waveguide along a straight line because of the surface tension of the photocurable adhesive 115. Accordingly, even if light is incident so that the incident position of the light spot (defined by the distance between the edge surface and the center of the light spot) in the direction of the longer diameter thereof is optimum, the coupling efficiency changes in accordance with the incident position of the light spot in the direction of the shorter diameter. Thus, it is difficult to precisely determine the incident position at which the coupling efficiency is maximum. Even if the incident position is determined at a position which is considered to be optimum, the maximum coupling efficiency cannot be necessarily obtained.
In order to solve the above-described problem, a structure shown in FIG. 12A has been proposed. FIG. 12B is an enlarged view of an edge surface E shown in FIG. 12A. In this structure, a light blocking layer 116 having an opening A is provided on the gap layer 113. The light blocking layer 116 has a refractive index lower than that of the waveguide layer 112. A sufficient amount of the photocurable adhesive 115 is applied to the inner wall of the opening A of the light blocking layer 116, and the prism 114 is pressed onto the photocurable adhesive 115. Then, light is radiated through the prism 114 to cure the adhesive 115. In such a structure, the photocurable adhesive 115 has the straight edge surface E along the inner wall of the opening A of the light blocking layer 116. Thus, the coupling efficiency is stabilized.
The "straight edge line" refers to a line, among the lines made by the side surface of the photocurable adhesive 115 crossing the gap layer 113 at substantially a right angle, which is perpendicular to the light axis and is located opposite to the light incidence surface (slanting surface) of the prism 114. The degree of straightness of the edge line along which the edge surface crosses the top surface of the waveguide is relatively determined with respect to the longer diameter of an incident light spot. For example, where the incident light spot has a longer diameter of about 10 .mu.m, the coupling efficiency is reduced to 90% of the maximum level when the incident position is offset by about .+-.2.5 .mu.m. Based on this fact, the term "straight line" in this specification refers to a line which is offset from the center line by about .+-.0.5 .mu.m or less.
In the above-described structure, it is difficult to adjust the amount of the photocurable adhesive 115 to fill the opening A. In the case where the amount of the photocurable adhesive 115 is excessively large, the photocurable adhesive 115 overflows around the side face of the prism 114 to flow to the section (not shown) supporting the prism 114. In the case where the amount of the photocurable adhesive 115 is excessively small, air bubbles remain in the opening A. Moreover, after the adhesive 115 is cured, the prism 114 is influenced by the stress accompanying shrinkage of the photocurable adhesive 115 while being supported by the light blocking layer 116. This presents a reliability problem. In an excessive case, the fatal phenomenon that the prism 114 is delaminated while the adhesive 115 is being cured may occur.